In recent years, researches and developments have been actively made in the field of materials comprising SiC as furnace inside parts in the production of silicon semiconductors or gallium nitride semiconductors and in the field of semiconductors comprising SiC as wide gap semiconductors. Owing to their high frequency, breakdown voltage, saturation drift velocity and thermal conductivity characteristics, SiC semiconductors have attracted attention as high efficiency and high voltage withstanding power devices and high-frequency devices, which are operable at high temperatures. SiC-coated carbonaceous materials, namely carbonaceous materials surface-coated with SiC, are used in fabricating SiC wafers, epitaxial grown layers, and devices which are necessary for the production of such SiC semiconductors.
So far, SiC-coated graphite materials have been used widely as tools or jigs used in the field of semiconductor production. Although attention has been paid to impurity concentrations as well, the scope of attention has been restricted to reduction in concentrations of heavy metals, in particular iron (cf Laid-open Japanese Patent Application (JP Kokai) No. 2002-128579 and JP Kokai 2002-128580).
Meanwhile, graphite materials generally contain metal impurities entrapped in pores and between lattice layers thereof, hence they as such cannot be used. Therefore, the Applicants have previously proposed, for use in the semiconductor and nuclear industries, high-purity graphite materials having a metal impurity (ash) contents of 5 ppm or less as a result of treatment of graphite materials with halogen-containing gases, for instance, for attaining high levels of purity (JP Kokai No. S64 (1989)-18986; Japanese Patent Publication (JP Kokoku) No. H06 (1994)-35325). They have also recently proposed, in JP Kokai 2002-249376, carbonaceous materials reduced in nitrogen content for use in the manufacture of compound semiconductors.
However, analysis by secondary ion mass spectrometry (hereinafter, “SIMS”) have revealed that even such SiC coated graphite materials still contain 1×1017 atoms/cm3 or higher of nitrogen and 4×1016 atoms/cm3 or higher of boron. It has also been revealed that this nitrogen penetrates into SiC on the occasion of producing epitaxial SiC layers and SiC devices and thereby increases the nitrogen concentration in SiC single crystals or SiC wafers and thus markedly deteriorates the semiconductor characteristics.
Furthermore, whereas the epitaxial growth of SiC is generally carried out at high temperatures of 1500° C. or higher, the SiC of the SiC coated graphite material used as tools is sublimated and the nitrogen and/or boron contained in the SiC coating is emitted. The nitrogen emitted from the SIC coated graphite material contaminates the epitaxial layer, so that the nitrogen content in the SiC epitaxial layer is 3×1015 atoms/cm3 or higher. This nitrogen increases the defect density of the epitaxial layer and at the same time functions as a dopant to the SiC semiconductor and deteriorates the device characteristics.
Graphite materials and SiC coated graphite materials are also utilized as tools or jigs in the production of silicon semiconductors and gallium nitride (GaN) semiconductors. It is known that, in the production of these semiconductors, contamination with boron, in particular, results in marked deterioration in semiconductor characteristics.
Further, nitrogen and/or boron contained in the SiC layer of the SiC coated graphite materials used as tools causes deterioration in semiconductor characteristics.